Disk files (or “drives”) are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head (or “transducer”) for reading or writing data onto the various tracks, and an actuator connected to the head for moving it to the desired track and maintaining it over the track centerline during read or write operations the movement of the head to a desired track is referred to as track accessing or “seeking”, while the maintaining of the head over the centerline of the desired track during read or write operations is referred to as track “following”.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to maintain the head precisely over the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. During track following, the servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a sampled position error signal (PES) which is an indication of the position error of the head away from the track centerline. The PES is used in the servo feedback loop to generate a control signal to the actuator to move the head back to the track centerline. A description of operation of a general disk file servo control system is given by R. K. Oswald in “Design of a Disk File Head-Positioning Servo”, IBM Journal of Research and Development, November 1974, pp. 506–512.
There are several causes for the head being off the track centerline during track following and which thus contribute to the PES. Certain position error components are of a low frequency, that is of a frequency close to the frequency of rotation of the disk, and are repeatable with disk rotation. For example, if the disk is not precisely centered over the axis of rotation of the spindle motor, the circular tracks will have an eccentric shape relative to the axis of rotation. This will cause a repeatable disk “runout” error at the same frequency as the rotation of the disk. Similarly, even if the disk is precisely mounted about the spindle motor axis, but the spindle motor axis “wobbles” during rotation, this will also generate repeatable errors. The problem of repeatable error is magnified in disk files which have multiple disks since each disk, and indeed each disk surface, will have its own unique repeatable error signature.
As recognized herein, another factor giving rise to RRO error is disk perturbation caused by screws that fasten the disk to the rotating spindle. As further recognized herein, this problem is further magnified near the inner portions of the disk. If the repeatable errors are known, correction signals can be fed forward into the servo control system to account for them. The present invention provides a solution to determine repeatable errors in a disk drive so that appropriate correction signals can be fed forward into the servo control system.